avoiding php emails going to spam or junk

We are having trouble with our emails being junked by gmail, hotmail etc when we send our users messages from a php mail() call. I have attached a basic outline of our emails... is there anything which might be jeapordising us, like the header info? And are there any definitie rules to help avoid our mails being junked?

Are you sending your emails from a known and checkable email address?
The first and second lines of your DOCTYPE are mismatched. And there was no </head> in your sample though you have just left that out.

If this is bulk email, services like Constant Contact, iContact, and Aweber do this very well... since it is their business and they spend all day doing it. See if you have any emails from them and look at the header formats and data they use.

When you’re making plans to join the modern business race, you should analyze various details that may affect your results. Nowadays, millions of businesses are trying to grow into established and appreciated professional enterprises.

There are times when I have encountered the need to decompress a response from a PHP request.
This is how it's done, but you must have control of the request and you can set the Accept-Encoding header.